Cook the pasta according to the package directions, drain, toss with just a touch of olive oil to prevent sticking, and set aside.
Bring a pot of water up to a rolling boil, add the cauliflower and cook until fork-tender, about 5 minutes, drain and set aside.
Toss together the white Cheddar and Roumy cheeses in a bowl, measure out ¾ cup and reserve it for the topping.
Add the oil to a medium large pot over medium-high heat. Add the onion and cook until softened, but not browned, about 5 minutes, stirring occasionally. Turn the heat down to medium and add the garlic and rosemary; cook 1 minute more, stirring constantly. Whisk in the flour and cook 1 minute, then gradually whisk in the milk; stir in the Worcestershire sauce, salt, and black pepper. Bring to a gentle boil, stirring constantly, and then remove from heat.
Whisk in the cheese, except the reserved amount) until smooth. Stir in the pasta, cauliflower, tuna, and parsley. Taste and season with additional salt and pepper as desired.
Preheat your broiler (counter top ovens, use the top coil) and butter a 9x13-inch baking dish. Pour the pasta mixture into the dish and sprinkle the reserved 3/4 cup of cheese on top. Broil until the cheese melts and is bubbly. Serve.